The present invention relates to the utilization of a corrosion proof austenitic iron-chromium-nickel-nitrogen alloy as structural material for components being subjected to high mechanical loads and being well amenable to welding. The chemical industry and engineering, for example, requires equipment and pressure vessel construction as well as devices for the production of energy use steel or alloys which are corrosion proof; can be welded without difficulty; and have sufficient strength comenserable with high mechanical loads. The 0.2% offset yield strength resp. the so-called 0.2 limit or yield strength resp. the yield point value are usually the requisite parameter for die calculations needed in the design. For this reason the construction engineer will prefer materials having very high 0.2% offset yield strength in order to attain restistance against highest possible load or because material and/or weight saving are required; also, thinner work pieces are easier to work and to weld. The development of such steel or alloys poses the difficult problem of maintaining or even attaining weldability of the material in spite of increased strength.
Contrary to ferritic steel, austenitic steel has quite favorable corrosion properties and is considerably better suited for welding, is more ductile and less brittle. Since nickel stabilizes the austenitic structure these steels have at least 7% nickel as for example reported in "Stahlschlussel" 13ed., 1983 "Stahlschlussel, Wegst GMBH Marbach, page 323 and 324 et sec. In order to obtain sufficient passivity this kind of steel has to have more than 16% chromium. In order to avoid intercrystalline corrosion the carbon content is limited to 0.08% particularly if the steel is not stabilized with titanium or niobium. A further improvement of the corrosion properties is attained through the addition of up to 6% molybdenum, up to 4% copper and up to 3% silicon. Higher nickel contents of about 50% improve the stress corrosion, see Berg- and HuenmaMonatshefte (BHM), 108,1963 pages 1/8 and 4 et sec.
The low guaranteed 0.2 limit of austenitic steel is stated in DIN 17 440 December issue of 1972 for steel for example from 18 to 19% chromium and about 9% nickel, to amount to 185 Newtons per square millimeter. The strength can be increased through solid solution hardening with up to 0.3% nitrogen to attain 343 Newtons per square milimeter. See also Japanese Industrial Standard JIS G4304, 19881 pages 1301/1304 et sec., SteelSUS 304 N 2. Such strength enhancement, however, does not meet all requirements.
In order to provide a further increase of the 0.2 limit it was required to introduce still more nitrogen, up to even app. 0.55% being the limit of solid solubility. Since nitrogen bubbles may accur during solidification forming blow holes in the ingot, and pores may appear during welding it is necessary to increase also the chromium and manganese content. Special steels are therefore known having from 22.5 to 25.5% chromium, from 4 to 7% manganese, from 2- to 4% molybdenum and from 13 to 17% nickel. In view of a content of nitrogen from 0.35 to 0.50% and in further view of a small amount of niobium as an additive, minimum values of the 0.2 limits are guaranteed from 500 to 540 newton per square millimeter. See also the ASM Technical Report 1970, No. C70-24.2 and the DEW Technical Reports 13, 1970, pages 94-100 and also Proceedings Molybdenum, 1973, Noranda Symposium 4, 1973 , pages 43-48.
These high alloyed special steels are indeed suited for welding just as the earlier mentioned common nitrogen alloyed austenitic steel. Their pure deposited weld metals are guaranteed for a 0.2 limit of at least 510 Newtons per square millimeter. However, these special steels are disadvantaged by the fact that the high chromium and nitrogen content renders hot working difficult. Moreover as temperatures as high as 1000 degrees centigrade intermetallic phases are precipitated which phenomena is responsible for low elongations less than 30%. Moreover, after welding hot straightening or bending a certain brittleness is observed. Since chromium in steel favors the formation of ferrite while nickel supresses the ferrite formation and also delays the precipitation of intermetallic phases it is not surprizing that these alloys have also a high nickel content which of course increases the cost of such a material once more.
Chemical engineering, however, requires usually relatively low alloyed steel having for example only about 18% chromium, 10% nickel and 2% molybdenum because such a steel is sufficiently corrosion proof, at least in most instances. Even a rather low 0.2 limit of such steel amounting to about 200 Newtons per square millimeter is tolerated and one dispenses with the addition of nitrogen because the nitrogen makes hot working somewhat more difficult while increasing the 0.2% offset yield strenth only to 280 Newtons per square millimeter. Compare for example Steel 1.4435 with Steel 1.4406 as per DIN 17440.
A wide utilization of common nitrogen alloyed austenitic steel has not yet occured even though the value of the 0.2 limit was further increased up to 343 newton per square millimeter. The same is true even for higher alloyed austenitic special steels with a nitrogen content above 0.35% and minimum value of the 0.2 limit of 500 Newtons per square millimeter. The utilization of this later type of steel is inherently limited to special instances and cases because of their high costs.
Another method for improving the strength property is grain-refining due to the formation of small grains. Thus cold working and subsequent recrystallization annealing of austenitic steel with approximately 18% chromium and 10% nickel yielded an ultrafine grained structure with grains of the size number 11.5 to 13.5 in accordance with ASTM and corresponding to 6 to 3 micrometers. See also ASTM Special Technical Publication No. 369 of 1965, pages 175-179. As compared with a rather coarse-grained inital state identically with the usual solution annealed condition of austenitic steels the 0.2 limit was increased by about 150 newtown per square millimeter. Since, however, the steel was not alloyed with nitrogen its 0.2 limit was still only, as an absolute value, about 380 Newtons per square millimeter. The problem therefore as far as such extremely fine grained steel is concerned and concerning any change and amenability to welding was not discussed in that paper.
The nitrogen alloyed austenitic steel as considered thus far is also to be considered with regard to the alloying element niobium. Its effectiveness is based on the precipitation of the complex nidtride of the kind Nb.sub.2 Cr.sub.2 N.sub.2 also called Z-phase. Even in hot worked solution annealed steels one obtains a grain size decrease which, however, is limited to grain sizes of No. 10 as per ASTM or corresponding 10 micrometers. See also BHM 142, 1979, page 513 et sec. In addition a certain nitride precipitation hardening was observed which increased the strength by 90 Newtons per square millimeter. See for example Thyssen Research Vol. 1, 1969, page 14 et sec. The precipitation of too much nitride has to be avoided because it extracts nitrogen from austenitic matrix as used for the solid solution hardening. In order to offset this effect these steels have a considerably smaller niobium content than their seven fold equivalent quantity of nitrogen which corresponds to the stoichiometric relation of the compound NbN.
The 0.2% offset yield strength at elevated temperature of austenitic steel is also usually increased through solid solution hardening and grainrefining. However, the increase of the 0.2 limit through the utilization of nitrogen will be lower with increasing temperature and for example at 400 degrees centigrade it is only half as large as at room temperature. See for example BHM 113, 1968, page 386 and 387 et sec. On the other hand the increase in the 0.2 limit attributable to grain-refining will decline considerably less with the test temperature as shown for example in Metal Science, Vo. 11, 1977 page 209. For still higher temperature the 0.2 limit is no longer determinative, but the somewhat lower, time dependent creep strength is decisive for design calculations. In this case the favorable small grain size effect is no longer effective.
A certain compensation can be provided through alloying with boron the alloy content being up to 0.015% because this feature increases the creep strength of austenitic chromium-nickel-molybdenum steel for temperatures of for example 650 degrees centigrade. See for example Revue Metallurgie 59, 1962, page 651/660. Even this kind of steel having additionally some nitrogen these favorable effects appear to be observed. See also Arch. Eisenhuttenwesen 39, 1968, page 146 et sec. and VDI Report 428, 1981 page 89 et sec. This way one increases the range of utilization under consideration of 0.2% offset yield strength at elevated temperature is to be considered in the calculations, and one can therefore shift the field of employment to still higher temperatures. However, it has to be observed that austenitic steel is prone to hot cracks during welding and for this reason the boron content is typically limited to 60 and 80 PPM.
Generally speaking the corrosion properties, particularly resistance against intercrystalline corrosion after welding are elaborated in DIN 17440, December issue of 1972. In particular austenitic steel alloy with up to 0.22% nitrogen is equated with steel without any nitrogen. They are both suited for welding if the wall thickness is smaller than 6 millimeters and has a carbon content below 0.07% while for thickness above 6 millimeters the carbon content even has to be below 0.03%. Only parts having 50 millimeter thickness as they are used in the pressure vessel engineering will have to be annealed after welding in accordance with the AD Flyer HP7/3 April issue of 1975.
The state of corrosion proof austenitic steel on delivery is usually determined by a treatment generally known as quenching. Basically it is a heat treatment and a healing process of at least 1000 degrees centigrade followed by very rapid cooling. This way all chromium carbides, and intermetallic phases will go into solution.
Moreover, the purpose of this feature is to remove dislocations appearing during working and as a result of deformation. These deslocations will be removed through recrystallization and recovery so that finally a state is obtained which has very low internal stress and, therefore, optimized corrosion resistance and ductility. However, one has to consider that in austenitic chromium-nickel-steel about 0.2% nitrogen and approximately 0.03% carbon are already in solution at 900 degrees centigrade. Therefore, annealing even at such relatively low temperatures in accordance with the remarks made above is still permitted if we are to make sure that for example cold worked steel will completely recrystallize at such temperatures and that before and after this heat treatment there are no intermetallic phases. Accordingly the pressure vessel engineering as per AD Flyer HP7/3 April issue of 1975 permits after cold working of nitrogen alloyed austenitic steel an annealing at 900 degrees centigrade in lieu of the quenching.
The welding connection of austenitic steel generally are evaluated by means of weld joint or weldment samples. These are flat samples in accordance with DIN 50 120 September issue of 1975 having a transverse welding seam which runs in the center and traverses the part in its entirety. Tear tests are conducted and make sure that the deposited weld metal, the parent metal and the metal in the small seam transition zone from weld to parent metal in the region of the fusion line are all subjected to the same force because they are arranged one behind another i.e., in a serial arrangement in the direction of the pulling force applied during the test. The sample and the method is in deed suitable for determining tensible strength and fracture position or location. However, it is of disadvantage that the elongation limits are ascertained only rather inaccurately by the method because the weld metal and the parent metal in the heat affected and unaffected zones will be plastically deformed differently strong within the measured length and will therefore differently extend in a permanent fashion. The fracture position in austenitic steel of usual grain size may occur in the parent metal and in the welding seam, while normally fractures are not to be expected in the seam transition zone. The strength properties are not ascertainable in this zone because they are simply too small. If a fracture occurs in the seam then of course the strength of the fusioned deposited weld metal itself is the deciding factor. Since the various filler metals are more or less fusioned with the parent metal, the tensile strength of the pure deposited metal, so-called all-weld-metal is determined separately in longitudinal samples with particularly prepared seams in order to have available sufficient reproducability of the data. In this case, no fusion occurs with the parent metal. The making of this type of longitudinal samples is described in DIN 32525, part 1, December issue of 1981.
The rate of fusion of the filler with the parent metal determined primarily by the electric current used for welding because that current determines the depth of the melted zone of the parent metal. Also the number of layers and the weld process itself are contributing factors to the rate of fusion. Furthermore, all features for reducing the overall heat input as such, and fast welding as in stringer beads low welding temperatures and avoiding the preheating are all advantage features.
In the case of a single-pass welding with the usual electric current the fusion rate are for example 20% in the tungsten-arc welding method, 30% in the manual arc welding method, 40% in the active-gas metal-arc welding method and 55% for submerged arc welding. In the case of multi-pass welding of rather thick cross sections there is a considerable reduction of this rate of fusion. On the other hand, welding of thin materials without filler metals the degree is of course 100%.
Suitability for welding of new steel is basically to be determined within the frame of so-called method tests. An important example in this connection and for austenitic steel is published in the AD Flyer HP 2/1 February issue of 1977 with the title "Method Testing of Weld Joints", (translated). This requirement refers primarily to the manufacture of test samples taken from steel welded by means of butt joints under certain conditions of manufacture so that for example parent metal, welding process, welding position, filler metals and auxiliary welding material are exactly determined. From the test sheets flat samples in accordance with DIN 50 120 are to be taken transversely to the seam, and the fracture position as well as the tensile strength is to be ascertained. The material is primarily deemed weldable if these weldment samples reach certain minimum value for the tensile strength of the effected parent metal or of the all-weld-metal, if fractures are located in the seam resp. weld metal.